PJ 2008 Ranger Auto o/d light flashes

Hi Everyone,
In the past few months the o/d light flashed on a reasonably warm day with vehicle on a long run. Problem happened on a long downhill stretch when I used the o/d switch for engine braking. Pulled over and let the trans cool, no change. Started up next day light didn't flash and has been OK up until now. Fault code through OBDI suggests "incorrect ratio on 5th gear". It did it again recently on a cold day and short run. Again OBDI code says same. I have investigated via Auto trans specialist, who will fix that problem for $2500! If its a more serious problem it could be a reco box for $5000+.
Also fault evident at over 200,00 kMs. Mine is at 131,00kM's. Is this a common problem?
Brian A